# Newtons 1st Law of motion in a vacuum

• aurao2003

## Homework Statement

Hi I was wondering what happens to a car accelerating in a vacuum?

## Homework Equations

Newtons 1st Law of motion: A Body continues at uniform speed or in a state of rest, unless acted upon by an external force

## The Attempt at a Solution

Outside a vacuum, cars will attain a constant speed due to air resistance eventually equalling the traction force. So, will Newtons 1st law negate in a vacuum? Since their is no air resistance, will a car built to 120mph exceed that speed? Just my mind meandering. Tell me what you think.

## Homework Statement

Hi I was wondering what happens to a car accelerating in a vacuum?
If it uses an internal combustion engine, it will stop accelerating.

Assuming an electric car, its top speed will be limited by the force of rolling friction. For a given traction force, it will keep accelerating until the forces opposing acceleration are equal to the force of traction.

AM

Well you have vacuum but still the car will have some friction from ground which will ultimately make it stop. if you even remove its contact with road then it will continue moving.

thats why the rockets don't require too much fuel when in space and meteors usually travel at speeds of 105m/s because there is no retarding force acting on them.

If it uses an internal combustion engine, it will stop accelerating.

Assuming an electric car, its top speed will be limited by the force of rolling friction. For a given traction force, it will keep accelerating until the forces opposing acceleration are equal to the force of traction.

AM
If rolling friction is eliminated and the impact from the ground, what can induce retardation? It seems the vehicle will continue (Not sure if it will be accelerating though).

yes .. it will continue to move but now it can't accelerate!

With friction eliminated will it remain in its motion perpertually? If yes, it seems we are back to Newtons laws of motion.

well i think it will ...

as long as remain on frictionless track, in vacuum and don't collide or experience any other natural force ... it will continue ...

With friction eliminated will it remain in its motion perpertually? If yes, it seems we are back to Newtons laws of motion.
Newton's Laws of Motion never leave.

AM

Newton's Laws of Motion never leave.

AM
So, Newton must have been 'The Man' of his generation, if not at all time. Can Newtons first law be demonstrated? Professor Walter Lewin stated in one of his lectures it cant. But then commented on its consistency and validity of results.

Can Newtons first law be demonstrated? Professor Walter Lewin stated in one of his lectures it cant. But then commented on its consistency and validity of results.
The difficult part of demonstrating Newton's first law is removing all forces: gravitational, frictional, electromagnetic, etc. However, it is possible to remove whatever forces can be removed and minimize the rest as much as possible. Consider that there are man-made space probes moving near the edge of our solar system, moving away from our solar system (e.g. Voyager missions). Some gravitational force (mostly from the sun) still acts of the space probes, but it's comparatively small. Friction from space debris is not completely gone either, but it is minimal. Whatever the case, the sum of the external forces on the spacecraft are quite small. And the spacecraft are headed away at approximately a constant velocity, just as Newton's laws would predict.

Perhaps a more testable law is Newton's second law, which states that an object's mass times its acceleration equals the sum of all forces acting on that object.

It's very testable to create a system where the sum of all forces on an object equals zero (for example, balancing gravitational force with an electrostatic force). If the sum of all forces is zero, it Newton's second law predicts the acceleration is zero -- meaning that the velocity remains constant. And in those situations, yes, the object keeps a constant velocity, just as Newton's laws predict. These are much more easily testable. If you continue studying physics, you will likely perform such tests yourself such as measuring the acceleration of a "car" on an air-track, which has very little friction. More advanced tests can be done too if you have the equipment.

The biggest problem in demonstrating Newton's first law acc. to me will come in choosing the inertial frame in which all forces ca be shown to be zero. And we can't overcome this problem by using pseudo force because (i) it is not a real force, and (ii) If a body is having a=0 in some ideally inertial frame then for this accelerating frame, it will be moving.
So first we need to find that "IDEAL INERTIAL FRAME" ... whose point of reference is still not fixed. I mean how do you define something not accelerating at all? you just define it not accelerating wrt something else. Is there something which can be treated as a fixed point for this whole universe and beyond? even if there is something like this then still we are very away from finding it.

The biggest problem in demonstrating Newton's first law acc. to me will come in choosing the inertial frame in which all forces ca be shown to be zero. And we can't overcome this problem by using pseudo force because (i) it is not a real force, and (ii) If a body is having a=0 in some ideally inertial frame then for this accelerating frame, it will be moving.
So first we need to find that "IDEAL INERTIAL FRAME" ... whose point of reference is still not fixed. I mean how do you define something not accelerating at all?
I pretty much agree with everything you're saying here, except for perhaps a couple of minor points, which I'll touch on below. But more importantly, you've given me another idea on how to practically test Newton's first law of motion. But first, a little background:

It is possible to test for acceleration on the absolute scale. Although such a test can only be performed the context of General Relatively (GR).

If you're in empty space, inside a laboratory where all the windows are closed (and always have been), there is absolutely no way for you to tell what your velocity is relative to anything else. Nor is there a way for you to tell what your position is relative to anything else. But you can tell what your acceleration is (see below, this is where GR needs to fit in). If you're accelerating you will feel push from one side of the laboratory -- similar to how you feel pushed around in an automobile when it is speeding up, breaking, or turning a corner quickly. But you don't need to rely on what you feel, you could place accelerometers at different points in the laboratory. If all the accelerometers read 0, the laboratory is not accelerating (according to GR, that is), simple as that. (And if some of the accelerometers are free-floating, placed stationary with respect to the rest of the laboratory, measure zero acceleration, and the distance between them doesn't oscillate, then there are no gravitational waves either. But now I'm getting too detailed.)

Now what about gravity, you might ask. According to GR, gravity is the fictitious force. A point particle free-falling in a gravitational field (any gravitational field) is not accelerating. Things get complicated here because our laboratory is not a point particle, it has spatial dimensions. But we can modify this principle by saying that a laboratory free-falling in a uniform gravitational field is not accelerating. Finding a uniform gravitational field is not practically possible, at least with our present technology. The non-uniformity in the gravitational field manifests itself as "tidal" forces.

The International Space Station (ISS) is an example of something that's very close to being in a perfect free-fall. It does experience tidal forces, as well as a touch of friction from the very upper part of the atmosphere, however these forces are comparatively tiny.

So the ISS is great place to test Newton's first law of motion, perhaps during a space-walk, assuming that we're all okay with putting up with the approximations involving tidal forces and residual atmospheric drag. And of course, assuming that we're all okay with putting everything in the context of general relativity.

As an astronaut (or cosmonaut) on the exterior of the ISS, holds a tool bag, still, in front of herself, and releases it, does it move away from her? No (at least it approximately stays still, considering our approximations). Now consider that she she gently pushes the tool bag away from herself and the space station. Once the tool bag is released from her pushing, does it move away from her at a constant velocity? Yes! Well at least approximately, given our approximations.

This tool bag scenario has [accidentally] been verified experimentally.
you just define it not accelerating wrt something else.
Well absolute acceleration needs no "definition." That is something that can be physically measured (within the confines of GR, that is.)

So an object moving at a constant velocity can be measured as not accelerating with respect to something else that is not accelerating, such as the non-accelerating laboratory (and remember, absolute acceleration can be measured).
Is there something which can be treated as a fixed point for this whole universe and beyond? even if there is something like this then still we are very away from finding it.
No, there is no such thing at all as a fixed point (absolute position) in the universe. It doesn't make much sense to even discuss that. Velocity is only relative too (not absolute). But absolute acceleration can be measured without invoking any concepts such as absolute velocity or absolute position. The only requirement is to do it within GR.

[Edit: You might not like my ISS/toolbag example at all, particularly since GR is way beyond Isaac Newton's time. And Isaac Newton had no idea how to measure lengths along GR's gravitational "geodesics." So I concede that my example might be more complicated than what it's worth. But you have to admit, it is a lot simpler and takes far fewer resources than physically putting a laboratory up somewhere, about half way between the Milky-way and Andromeda galaxies. ]

Last edited: